Investigation On Predictive Modeling Of Thermophysical Properties And Thermal-hydraulic Characteristics Of Nanofluids

As a kind of special liquid-solid mixture,nanofluids have attracted great attentions due to its higher thermal conductivity and heat transfer performance.In recent years,many investigations found that affected by the factors including preparation process,environment and measuring technology,the preparation method of nanofluid was still immature.And the stability of nanofluids in the practical application was very outstanding,which limited the development of nanofluids.In addition,considering the effects of small size characteristics of nanoparticles,the heat and mass transport mechanisms of nanofluids are so complex those the thermal conductivity and viscosity cannot be decribed by using the tranditional liquid-solid transport theory.As a result,the flow and heat transfer characteristics of nanofluids has not been fully clear.Therefore,it is very necessary to carry out more investigations to study the preparation stability of nanofluids,explain the transport mechanism of nanoparticles under the effects of different factors and further understand the heat and mass transfer performance of nanofluids.With the applied background of nanofluids in the plate-fin heat exchanger,this paper focuses on the analysis of nanofluids preparation stability,the modeling of thermophysical properties and the thermal-hydraulic performance of nanofluids in an offset strip fins channel.The detailed contents are shown in the following.(1)This research prepared Al2O3-H2O nanofluids by using the "two-steps" method,and discussed several factors that affecting the stability of nanofluids.The results showed that ultrasonic process and dispersant were important factors to improve the uniformity of nanoparticles and the stability of nanofluids.Besides,the effects of dispersant were more obvious than that of ultrasonic process.(2)An experiment was conducted to study the effects of nanoparticle volume fraction,nanoparticle size and temperature on the thermal conductivity and viscosity of nanofluids.On the basis of this,this paper proposed a data-driven modeling approach for predicting the thermophysical properties of nanofluids.It was found that the thermal conductivity and viscosity could be enhanced obviously with the increases of nanoparticle volume fraction and temperature and the decrease of nanoparticle size(the increase of ultrasonic processor time).In addition,the case study demonstrated that RBF neural network was an effective approach for the thermal conductivity and viscosity prediction of nanofluids with sufficient samples.Not only did the predicted results of RBF neural networks agree well with the experimental data,but also the effects of different factors could be reflected.(3)To solve the prediction problem of RBF neural networks with small samples,the theoretical models of nanofluids thermal conductivity and viscosity were respectively presented by using mechanism modeling and experimental data correction.According to the results of example verification,it was found that both the interface layer and the Brownian motion of nanoparticle could enhance the thermal conductivity and viscosity of nanofluids.The new models developed in this paper exhibited better prediction performance than many existing theoretical and empirical models.(4)Using the single-phase based numerical approach,this paper studied the three-dimensional laminar flow and heat transfer behavior of water based nanofluids in an offset strip fins channel.Parametric variations were analyzed for explaining the influences of different nanoparticle properties and Reynolds number.On the basis of these,the new correlations for the Colburn factor and Fanning factor as a function of particle volume concentration,Reynolds number and Prandtl number were developed.The numerical results indicated that both the heat transfer and pressure loss of offset strip fins channel were affected significantly by the volume fraction,size and type of nanoparticles.However,the influence mechanism of every factor was different.At low Reynolds number,the addition of higher volume fraction and smaller Al2O3 nanoparticle was more benefit on the improvement of heat transfer.While considering the flow characteristics of nanofluids,the overall performance could be enhanced by decreasing the nanoparticle volume fraction,increasing the size and thermal conductivity of nanoparticle.(5)Considering the comprehensive effects of nanofluids in offset strip fins channel,a criterian formula was given for evaluating the enhanced single-phase convective heat transfer of nanofluids.After the numerical experiments,it showed that the classical evaluation indexes and assumption were not completely suitable for the nanofluids due to the changing of thermal physical parameters.The comprehensive performance evaluation criteria established in this paper could effectively reflect the effects of volume fraction,size and type of nanoparticles on the thermal-hydraulic performance of nanofluids.According to the comprehensive evaluation,the study revealed that with the increment of nanoparticle volume fraction and the decrement of nanoparticle size,the super advantage of nanofluids reduced gradually.For the present investigations,the application potentiality of nanofluids was more obvious at the same pumping power.